Metal strength members, reinforcing members, or structural members which are used in automobiles and various types of equipment and which have a bent shape (referred to below as bent members) need to have a high strength, a low weight, and a small size. In the past, bent members have been manufactured by, for example, welding of press-formed members, punching of plates, or forging. The extent to which a bent member achieved by these manufacturing methods can be made lighter or smaller in size has reached a limit.
In recent years, the manufacture of bent members by the so-called tube hydroforming technique has been actively studied (see Non-Patent Document 1, for example). On page 28 of Non-Patent Document 1, it is stated that there are various problems associated with the tube hydroforming technique such as developing materials to be processed or increasing the degree of freedom of the shape which can be formed whereby further technical development of the tube hydroforming technique is necessary.
Besides, there have been many disclosures of techniques for manufacturing a bent member by bending a straight metal tube.
Patent Document 1 discloses a technique for bending a metal tube while subjecting it to heat treatment. Patent Document 2 discloses a technique for manufacturing a curved spiral bar or section having a shaped cross section. Patent Document 3 discloses a bending apparatus which utilizes high frequency induction heating. Patent Document 4 discloses a bending apparatus for a metal member.
In Patent Document 5, the present applicant disclosed a bending method and a bending apparatus for a metal blank. FIG. 19 is an explanatory view showing an outline of this bending apparatus 0.
As shown in FIG. 19, a steel tube 1 is supported by a support means 2 so that the steel tube 1 can move in its axial direction. A feed device 3 has a ball screw, for example, as a component. The feed device 3 feeds the steel tube 1 from an upstream side to a downstream side.
An induction heating coil 5 is disposed downstream of the support means 2. The induction heating coil 5 rapidly heats a portion of the steel tube 1 by induction heating to a temperature range in which quench hardening is possible. A water cooling device 6 is disposed downstream of the high frequency heating coil 5. The cooling device 6 rapidly cools the steel tube 1. As a result, the temperature of the steel tube 1 becomes a temperature at which quench hardening is possible only between the induction heating coil 5 and the water cooling device 6 (below, a portion of the steel tube 1 which is at a temperature at which quench hardening is possible will be referred to as a high-temperature area).
A movable roller die 4 is movably disposed on the downstream side of the water cooling device 4. The movable roller die 4 has at least one set of roll pairs 4a. The roll pairs 4a can support the steel tube 1 while feeding it. The movable roller die 4 moves two-dimensionally or three-dimensionally. As a result of the movement, a bending moment is imparted to the high-temperature area of the steel tube 1 so that the steel tube 1 undergoes bending. In this manner, a bent member 8 is manufactured with a high operating efficiency while maintaining sufficient bending accuracy.